Effects of heat stress


“Heat waves in India takes a large number of deaths every year.”
– R. R. Nair (R. Rajasekharan Nair)


World climate is changing gradually due to global warming. Global warming is the term used to describe a gradual increase in the average temperature of the Earth’s atmosphere and its oceans. The average temperature of the earth has risen between 0.4 oC and 0.8 oC over the past 100 years. Scientists from the Intergovernmental Panel on climate carrying out global warming research have recently predicted that the average global temperature could increase between 1.4 oC and 5.8 oC by the year 2100.

There is a rise in temperature ranging from 2 oC to 3 oC during summer in many parts of the India. This increase in temperature pose many health problems to flora and fauna. According to a report, India is now on a path to between 2.2 oC and 5.5 oC of temperature rise by the century and the rate of heat wave deaths in India and other Asian countries could soar. Heat waves in India are taking a large number of deaths in every year. According to National Crime Records Bureau (NCRB), between 2010 and 2015, a total of 7686 people lost their lives in India due to heat stroke, i.e. an average of 3 deaths per day.

It may be recalled here that in May 2015, India was struck by a severe heat wave, which has caused the deaths of at least 2500 people in multiple regions. Out of these, the state of Andhra Pradesh and Telangana accounted the deaths of 1735 and 585 people respectively.

In India, the heat wave occur during the summer, which typically lasts from March to July, with peak temperature in April and May. Although it typically remains hot until late October, Indian monsoons often provide some respite from the heat.

The World Health Organization predicts that heat-stress, linked to climate change, is likely to cause 38,000 extra deaths a year worldwide between 2030 and 2050.

The main focus of this article is about the ill-effects of heat on human body. However, before discussing the ill-effects, it would be better to understand the basics of metabolism, energy exchange, energy balance, thermal environment measurements and body reactions in climates. The article also focus on microclimate and thermal comfort.


The body of a working adult may continuously generate heat from the food he eats, varying from about 2200 K calories per 24 hours for sedentary mental work to 4000 K calories or more per 24 hours heavy work while standing. The corresponding figures for a female are 15% to 20% less. The efficiency of the body as a heat engine is about 20%, so that for every unit of mechanical work done, 4 units are converted to heat and have to be removed.

In human body, a temperature regulating centre in the brain maintains the temperature of the brain, the heart, and the abdomen at about 37 oC, with a swing of about 0.5 oC between night and day. The temperature of the skin and body extremities is lower and varies considerably, from about 30 oC to 35 oC. While there is some fluctuations throughout the day due to diurnal changes in body functions, the main impact upon the human thermal regulatory system, results from the interaction between the heat generated within the body and external energy gained in hot surroundings or lost in a cool environment. If the deep body temperature deviate just a few degrees from its set value, physical and mental work capacities are impaired.

If the temperature of a human cell exceeds 45 oC, heat coagulation of proteins take place, but if the temperature reaches freezing, ice crystals break the cell apart. In order to protect itself from conditions that are either too hot or too cold, the human temperature regulation system must keep temperatures well above freezing and below the 40 oC in its outer layers. At the core, a range close to 37 OC must be maintained. The changes in core temperature plus or minus 2 oC from 37 oC affect body functions and task performance severely. The deviations of plus or minus 6 oC are usually lethal.


Energy is exchanged with the environment through radiation (R), convection (C), conduction (K) and evaporation (E).

Heat loss by radiation depends on the difference between the skin temperature and that of the surrounding surfaces. Heat is always radiated from the warmer to the colder surface. Therefore, the body can either loose or gain heat through radiation. In temperature climates, 40 to 60 % of our body heat is lost in this way. When the surrounding surfaces are hot, however, little heat can be lost in this way and more has to be lost by perspiration.

Heat loss by convection depends on the air temperature and movement, the clothing worn and the percentage of the body that is exposed. It normally amounts to 25 to 30 % of the heat loss from the body to the environment, but in higher, in cold and windy weather.

Heat loss by conduction depends on the conductivity of the surroundings. Conductance exists when the skin contacts a solid body, such as piece of iron. Energy flows from the warmer body to the colder one; as the temperatures of the contact surface become equal, the energy exchange ceases. The rate and amount of heat exchange also depend on the conductance of the touching bodies.

Normally about One litre per day of water evaporates from the skin and respiratory systems, accounting for about 25 % of the body’s heat loss. But at temperature above 25 oC, when less heat can be lost by convection and radiation, more has to be lost by perspiration and evaporation, by stimulation of the sweat glands. Therefore, heat balance exists when metabolic energy ‘M’ developed in the body, heat storage ‘S’ in the body, and heat exchanges with the environment by radiation ‘R’, convection ‘C’, conduction ‘K’ and evaporation ‘E’ are in equilibrium.

This can be expressed as: M+S+R+C+K+E=0

The quantities ‘R’, ‘C’, ‘K’ and ‘E’ are negative, if the body loses energy to the environment and positive, if the body gains energy from the environment. ‘E’ can only be negative.

In a hot environment, body heat must be desiccated and gain from the environment be prevented. This is primarily done by increased blood flow to the skin, by sweat production and evaporation.


The human body must regulate its temperature to prevent undercooling or overheating. The body temperature is not all uniform. There are large number of temperature difference between the ‘Core’ and the ‘Shell’. Under normal conditions, the average gradient between skin and the deep body is about 4 oC at rest, but in cold, the difference in temperature many be 20 oC or more. In reality, the temperature regulation system has to maintain various temperatures at various locations under different conditions.

The human body has given set point near 37 oC in the brain & about 33 oC at the skin. Any deviation from this values are detected by various sensors and counter actions are initiated at Hypothalamus. The major avenues of the human body to control heat transfer between core and skin are the different motor pathways (muscle tonus), sudomotor pathways (sweat production) and vasomotor pathways (control of blood flow). For example, if less heat must be generated internally, muscular activities will be reduced, possibly to the extent that no work is being performed anymore. On the other hand, if more heat must be generated, the work or exercise level will be augmented by increased muscular activities. However, it may be remembered here that the muscle activities can generate only more or less heat but cannot cool the body. In contrast, sweat production only influences the amount of energy lost but cannot bring about a heat gain. Vascular activities can affect the heat distribution through the body and control heat loss or gain, but they do not generate energy. Muscular, vascular and sweat production functions regulate the body heat content in direct interaction with the external climate.

Muscular activities are the major means to control heat generation in the body. Blood flow control affects heat transfer between body core and skin.

If heat gain is to be achieved, skeletal muscle contractions are initiated; but if heat loss is desired, muscular activities are abolished. Loss is also achieved through regulation of skin blood supply, sweat production and changes in clothing and shelter. If heat gain must be prevented, clothing and shelter are adjusted. If heat loss must be prevented vasoconstriction methods at the skin will be used, and clothing and shelter will be altered.

Changes in the clothing and shelter are to achieve thermal homeostasis. They affect radiation, convection, conduction, and evaporation.

Clothes affect conductance. Also their colour determines how much external radiation energy is absorbed or reflected. Similar effects are brought about by shelters, which by their material, the distance from the body, form and colour, determine whether heat is gained or lost by the body through radiation, convection and evaporation.


The thermal environment is determined by four physical factors viz.: (i) Air Temperature, (ii) Air Humidity, (iii) Air Movement, and (iv) Surface Temperature.

The combination of these four factors determines the physical conditions of the climate and our perception of the climate.

5.1 Air Temperature:
Measurement of Air Temperature is performed with thermometers, usually filled with alcohol or mercury. While measuring, it must be ensured that the ambient temperature is not affected by the other three climate factors viz., humidity, air movement and surface temperature. To measure the dry temperature of ambient air, one keeps the sensor dry and shields it with surrounding bulb that reflects radiated energy. Air temperature is often measured with ‘dry bulb’ thermometer.

5.2 Air Humidity:
Air humidity is measured with a psychrometer, hygrometer or other electronic devices. Air humidity may be expressed either in absolute or in relative terms.

5.3 Air Movement:
Air movement is measured with various types of anemometers, usually based on mechanical or electrical principles. Air movement can also be measured with 2 thermometers – one dry and one wet (similar to what can be done to assess humidity) relying on the fact that the wet thermometer shows more increased evaporating cooling with higher air movement than the dry thermometer.

5.4 Surface Temperature:
Radiant heat exchange depends primarily on the difference in the temperature between the individual and surroundings, on the emission properties of the radiating surface, and on the absorption characteristics of the receiving surface. To assess the amount of energy transferred through radiation is, to place the thermometer inside a black globe, which absorbs practically all radiated energy.

Various techniques exists to express the combined effects of the four environmental factors in one module, chart, or index. One method used is Effective Temperature Scale. Alternatively the readings may be used along with an assessment of energy expenditure to derive a single expression of physiological strain, such as the Predicted Four Hour Sweat Rate (P4SR) or Belding – Hatch index. Another method is the Wet Bulb Globe Temperature (WBGT) index which weighs the effects of several climatic parameters.


The body produces heat in hot climates, which must desiccate it. Similarly, the body must conserve heat in a cold climate. For this there are ways to regulate the temperature, which are briefed below:

6.1 In Hot Climate:
In hot climate, the body produces heat and must desiccate it. Two primary means exist to control the energy flow: blood distribution and metabolic rate. To achieve this, the skin temperature should be near, best above the immediate environment. Blood is redistributed to allow heat transfer to the skin. For this, the skin vessels are dilated and superficial veins are fully opened. This may bring about a four-fold increase in blood flow above the resting level, increasing the conductance of the tissue. Accordingly, energy loss through convection, conduction and radiation is facilitated. If the heat transfer is still not sufficient, sweat glands are activated and evaporation of the produced sweat cools the skin. The overall amount of sweat developed and evaporated depends very much on clothing, environment, work requirements and on the individual acclimatisation. If heat transfer by blood distribution and sweat evaporation is insufficient, muscular activities must be reduced to lower the amount of energy generated through metabolic processes. In fact, this is the final action of the body, if otherwise the core temperature would exceed a tolerable limit. In other words, the ultimate aim of the body is to maintain the core temperature, which means reduction or cessation of work activities.

6.2 In Cold Climate:
In cold climate, the body must conserve heat while producing it. For this there are two major ways to regulate the temperature, i.e. the distribution of the blood flow, and increase in metabolic rates. To conserve heat, the temperature of the skin is lowered to reduce the temperature differences against the outside. This is done by displacing the circulating blood towards the core, away from the skin. Blood distribution can be regulated by three procedures, viz.: (i) constriction of skin vessels, (ii) use of deep veins and (iii) increased heat exchange between arteries and veins. In a resting individual highly clad, in an ‘ideal’ external temperature of about 28 oC, the mean skin temperature is about 33 oC & core temperature is about 37 oC.

As the skin temperature is lowered to about 15 oC to 20 oC, manual dexterity begins to be reduced. Tactile sensitivity is severely diminished as the skin temperature falls below 8 ºC. If the temperature approaches freezing, ice crystals develop in the cells and destroys them, as a result known as ‘frostbite’.

Reduction of core temperature is more serious, where vigilance may begin to drop at temperature below 36 oC. At about 35 oC, one may not be able to perform even simple activities. When the core temperature drops even lower, the mind becomes confused with loss of consciousness occurring around 32 oC. A core temperature of about 26 OC, heart failure may occur. At core temperature of about 20 oC, vital signs disappear, but oxygen supply to the brain may still be sufficient to allow revival of the body from Hypothermia. It may be kept in mind that severe reduction in skin temperatures are accompanied by a fall in core temperature too. Hypothermia can occur very quickly if a person is exposed to cold water. While one can endure up to two hours in water at 15 oC, one is helpless in water at 5 oC after 20 to 30 minutes. The survival time in cold water can be increased by wearing clothing which provides insulation.


There are several signs of excessive heat strains in the body. The first one is the sweat rate. On the average during working hours, usually not more than about one litre per hour is produced. However, sweat losses up to 12 litres in 24 hours have been reported under extreme hot conditions.

Increases in the circulatory activities will be one of the signs of heat strain. Another sign is higher heart rate, which will result in enlarged cardiac output. This may be associated with a reduction in systolic blood pressure. Rise in core temperature will be another sign of heat strain. The water balance within the body provides another indication of heat stress. Dehydration indicated by the loss of 1 or 2 % of body weight can critically affect the ability of the body to control its functions. The heat stress may also have physiological ill effects. The harm done is indirect and manifests itself in high accident rates, increased sickness among workers and lowered productivity.

Among the first reactions to heavy exercise in excessive heat are sensations of discomfort and perhaps skin eruptions (prickly heat) associated with sweating to a rise in body temperature above 37.5 oC.

Heat stress and heat disorders can lead to many ailments, and some of them are discussed below:

7.1 Sun Burn:
Sun burn happens when the skin absorb too much sunlight. This can be harmful. Mild sun burn can be treated at home, e.g. by applying moisturiser. Some sun burn may need medical attention.

7.2 Heat Rash:
Heat rash is caused by exposure to warm temperature. It is an uncomfortable skin conditions that happens when obstruction causes sweat to leak into the deeper layers of skin. If perspiration is not removed from skin, sweat glands will be inflamed which in turn results in heat rash or prickly heat. Heat rash or prickly heat is also known as miliaria rubra, summer rash, and wild fire rash. The symptoms of heat rash are rash in area of heavy perspiration, discomfort or temporary disability. There may be warm stinging feeling and a rash of small red dots. It usually clears up on its own after few days. It often affects children and infants. Treatment of heat rash include periodic rest in a cool area, showering/bathing and drying skin.

7.3 Heat Cramps:
As a result of sweating, heat cramps may develop, which are muscle spasms related to lack of salt in the body.

7.4 Heat Syncope:
Heat syncope indicates a failure of circulatory system, demonstrated by fainting. Fainting will occur due to shortage of oxygen in the brain. The notable symptoms of heat syncope are black out and collapse. The best treatment for heat syncope is lay down.

7.5 Heat Exhaustion:
Heat exhaustion occurs, if the body is dehydrated and is unable to regulate its internal temperature. Heat exhaustion is a combined function of dehydration and overloading of the circulatory system. The main symptoms of heat exhaustion are extreme weakness or fatigue, dizziness, giddiness, nausea, muscle cramping, rapid weak pulse, dark coloured urine, headache, pale of flushed complexion, moist skin, etc. Generally body temperature is normal or slightly high. In extreme cases of heat exhaustion, vomiting and/or loss of consciousness may occur. The main causes of heat exhaustion are loss of water and/or salt in the body, loss of blood plasma, strain on the circulatory system, etc. The best treatment suggested for heat exhaustion is rest in cool area and consuming salted liquids, unless advised differently by a physician. It may be remembered that without treatment, it can develop into heat stroke, a potentially fatal condition.

7.6 Heat Stroke:
Heat Stroke or Sun Stroke or Siriasis is a type of severe heat illness that results in a body temperature greater than 40 oC  (104 OF). It indicates an overloading of both the circulatory and sweating systems. The main causes of heat stroke are the breakdown of the thermo-regulatory system under stress, which results in the stoppage of sweating. The body’s ability to remove excess heat is almost eliminated. The main symptoms associated with heat stroke are, skin hot, dry and often red or spotted; core temperature of the body might be 40 oC (104 OF) or higher and rising. Mental confusion, throbbing headache, dizziness and light headedness, lack of sweating despite the heat, muscle weakness or cramps, nausea and vomiting, rapid heartbeat which may be either strong or weak, rapid or shallow breathing, deliriousness, convulsions and possible unconsciousness may occur. It can lead to a life threatening medical emergency. If not treated it can damage multiple organs and systems. Death or permanent brain damage may result unless treated immediately. In severe cases, this can lead to coma and if the body temperature reaches 42 oC, death usually follows within 24 hours.

There are two types of heat stroke: exertional or non-exertional. Older adults, people with chronic illness, and infants are often affected by non-exertional heat stroke. A person typically experiences this type of heat stroke, when they are indoors without air-conditioning, and they may not be engaging in any physical activity. It can take several days of high temperature for non-exertional heat stroke to occur, and it is common during extreme heat waves. Exertional heat strokes occurs in people, whose bodies can no longer adopt to rising temperature, while exercising or working. This condition can develop within a few hours, usually people who are at outdoor.

It may be kept in mind that spending time in closed cars put small children at high risk of heat stroke. As per an estimate, when the temperature outside is 80 OF (26.66 oC), the temperature inside closed car rises to 109 OF (42.77 oC), within 20 minutes. The hotter it is outside, the faster the temperature rises inside a vehicle.

The best treatment of heat stroke is to remove the victim to cool area, soak clothing with cold water, fan body, and call a physician/ambulance immediately. To prevent heat stroke, drink plenty of water, especially when exercising, take cold baths or showers, wear light coloured loose clothing, sprinkle water over skin or clothes, avoid sun between 11 AM and 3 PM, avoid excess alcohol and avoid extreme exercises. With heat stroke, the body temperature might be more than 103 OF (39.44 oC) to 104 OF (40.00 oC), depending on a person’s normal average body temperature.

7.7 Hyperthermia:
Hyperthermia is a condition that results in an abnormally high body temperature. It occurs when the body can no longer release enough of its heat to maintain a normal temperature. The body has different coping mechanisms to get rid of excess body heat, largely breathing, sweating and increasing blood flow to the surface of the skin. But when the environment outside is warmer than the inside of the body, the outside air is too warm or humid to passively accept heat from the skin and evaporate sweat, making it difficult for the body to release its heat. An overheating progress, more and more moisture and electrolytes are lost from the body, lowering blood pressure and limiting sweating. Hyperthermia can affect people who work or play sports in a very hot environment. It can lead to dangerous and potentially fatal complications. Hyperthermia is also more likely to cause complications in people with heat related heart and blood pressure conditions. It may be reminded here that in human, core body temperature ranges from 95.9 OF (35.5 oC) to 99.5 OF (37.5 oC). In contrast, people with some level of hyperthermia have a body temperature of more than 100.4 OF (38 oC). A body temperature of more than 104 OF (40 oC) is defined as severe hyperthermia. Heat exhaustion is one of the more serious stages of hyperthermia.


A suitable microclimate is highly individual and also variable. It depends on gender and age.

Thermal comfort depend largely on the type and intensity of work performed. Physical work in the cold climate may lead to increased heat production and hence to less sensitivity to the cold environment, while in heat, hard physical work could be highly detrimental to the achievement of an energy balance.

Overheating causes sensation of tiredness, making performance more tedious and increasing the frequency of errors. Overcooling results in restlessness and reduced attention which particularly affect mental work. While people can gradually adopt to a wider temperature range, this creates stress in their regulatory mechanism and they are less comfortable, working less safely and effectively.

With regard to airspeed and comfort, an airspeed of up to 0.5 meter/second are suitable for standing work. Airspeeds greater than 0.2 meter/second feel uncomfortable to a seated persons and shall be avoided in offices. For every precious work, airspeed should not exceed 0.1 meter/second.

The relative humidity is very important. Relative humidity below 30% cause dehydration of mucous membranes and respiratory tracts, and reduce resistance to cold and influenza. They also encourage the formation of static electricity and may cause damage to wooden furniture.

Clothing largely affects the microclimate. Clothing also determine the surface and of exposed skin. Thermo-comfort is obviously also affected by acclimatization, i.e., the status of the body and mind of having adjusted to changed environment conditions.

With appropriate clothing and light work, comfortable ranges of effective temperature are about 21 oC to 27 oC Effective Temperature in a warm climate in summer and 18 oC to 24 oC Effective Temperature in a cool climate in winter. In terms of body measurements, skin temperature in range of 32 oC to 36 oC are considered comfortable, associated with core temperature between 36.7 oC and 37.1 oC. Preferred ranges of relative humidity are between 30% and 70%. Deviation from these zones are uncomfortable or even intolerable.


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Article by —–

Mr. R.R. Nair, Chief Executive, Safety and Health Information Bureau, Vashi, Navi Mumbai
Mr. R.R. Nair,
Chief Executive,
Safety and Health Information Bureau,
Vashi, Navi Mumbai


Mr. R. R. Nair (R. Rajasekharan Nair) is currently the Director of Safety and Health Information Bureau. He is an ex-employee of Central Labour Institute, DGFASLI, Mumbai, and retired from the Government Service after 28 years. He had undergone specialised training in Occupational Health & Safety (OHS) at ILO-CIS, WHO, HSE (UK) and RoSPA (UK). He has more than 50 years’ experience in OHS & Fire Protection. He had represented India at the 23rd meeting of ILO-CIS held at Geneva. He has also participated in a number of seminars, conferences, workshops on OHS & Fire Protection at National levels. He has carried out about 85 projects in safety, health, environment and fire protection (safety audits, accident investigations, environmental studies, hazard identification and risk assessment (HIRA), hazardous zone classifications, fire safety audits in high rise buildings, etc.) PAN India. He is author of 15 books and about 100 articles in various topics on safety and allied subjects.

He can be contacted on:
M: +91 7045172050, +91 9224212544
Resi: +91 477 2266994
E-mail: [email protected] / [email protected]
Website: www.shib.co.in